Addis Ababa: South Sudan's warring parties began negotiations on Friday to end nearly three weeks of raging conflict which has left thousands feared dead and taken the world's youngest nation to the brink of all-out civil war.

Fighting intensified as the army moved on a key rebel-held town, even as government and rebel negotiating teams gathered at a luxury hotel in neighbouring Ethiopia's capital Addis Ababa.

"We have enough forces who will defeat the rebels within 24 hours," army spokesman Philip Aguer said in South Sudan, with reports of heavy battles involving tanks and artillery on the outskirts of Bor, a dusty town that has already exchanged hands three times since fighting began.

"These forces -- the rebels -- are now retreating back," Aguer said, quashing rebel claims that they themselves had been marching on the capital Juba.

The US embassy in South Sudan ordered a further pullout of staff and urged all citizens to leave on an evacuation flight it had organised because of the "deteriorating security situation."

The ongoing battles prompted the top UN aid official in South Sudan, Toby Lanzer, to warn that soldiers and rebels must protect civilians and aid workers, or risk worsening a situation he described as "critical".

But in the calm of the hotel in Addis Ababa, rivals met special envoys from regional nations, ahead of direct talks that sources suggested may not take place until at least Saturday.

Ethiopia's foreign ministry said the regional East African bloc IGAD that is helping to broker a deal "was committed to support in any way possible".

Thousands of people are feared to have been killed in the fighting, pitting army units loyal to President Salva Kiir against a loose alliance of ethnic militia forces and mutinous army commanders nominally headed by ex-vice president Riek Machar.

Fighting erupted on December 15 when Kiir accused Machar of attempting a coup in the oil-rich but impoverished nation.

Machar denied this, in turn accusing the president of conducting a violent purge of his opponents and refusing to hold direct talks with Kiir.

Civilians in dire need of aidFighting has spread across the country, with the rebels seizing several areas in the oil-rich north.

Aid workers have increased warnings of a worsening crisis for civilians affected by the conflict in the landlocked country of almost 11 million people.

"All parties to the conflict have a responsibility to make sure that civilians are spared from the fighting," said Lanzer, the UN humanitarian coordinator.

The violence has forced around 200,000 people to flee their homes and "affected many hundreds of thousands of people indirectly", Lanzer added.

Some 57,000 are seeking refuge with badly overstretched UN peacekeepers.

The UN peacekeeping force said this week "atrocities are continuing to occur" across the country, which won independence from Sudan in 2011 after decades of civil war.

One of the hardest hit areas is Bor, the capital of Jonglei state and situated just 200 kilometres (125 miles) north of the capital Juba.

Tens of thousands have fled, many paddling in simple boats across the crocodile-infested White Nile river to escape the fighting.

The conflict has been marked by an upsurge of ethnic violence pitting members of Kiir's Dinka tribe against Machar's Nuer community.

The United Nations reported "extra-judicial killings of civilians and captured soldiers" and the "discovery of large numbers of bodies" in the capital Juba as well as in the towns of Bor and Malakal.

The army has set up committees into the killing of "innocent people", the government said Thursday, and another into the bitter infighting within the presidential guard units that were the reported first shots in the conflict.

Machar told AFP on Tuesday that he was not yet ready to agree to an immediate ceasefire nor hold face-to-face talks with Kiir.

Kiir has described the war as "senseless", but has ruled out power sharing with the rebels.

"If you want power, you don't rebel so that you are awarded with the power," Kiir said in an interview broadcast on the BBC.